This invention relates generally to signal converters and is specifically directed to a signal converter particularly adapted for use in the widely varying, low signal level, large bandwidth environment encountered in extended bandwidth CATV systems.
A frequency converter, or translator, ideally processes a signal by frequency conversion and amplification, without significantly altering any signal characteristic except the amplitude and frequency. Frequency translation, either upward or downward, is a common technique used in many communication and navigation systems to take advantage of optimum signal processing characteristics of the equipment used in the system. The upward or downward conversion of the incoming RF signal to another RF frequency is accomplished by mixing the incoming RF signal with a reference frequency signal provided by a local oscillator to produce either an upconverted and/or a downconverted RF signal for subsequent processing in the receiver.
Now that cable television (CATV) is expanding rapidly into systems that will cover increasingly larger geographical areas with the increased demand for additional services and thus more channels, the need for techniques to improve or maintain quality performance is evident. Going from 300 MHz (35 channels) to 440 MHz (60 channels) or 500 MHz (70 channels) or even to 550 MHZ (77 channels) significantly increases the number of interference causing channels and degrades the composite triple beat performance in addition to increasing second order intermodulation and cross-modulation distortion. Also, extending to the higher frequencies increases cable losses in proportion to the square root of the frequency and a wider range of signal levels will more likely be encountered than the signal levels for narrower bandwidth 300 MHz CATV systems.
In order to maintain or improve the performance of 300 MHz CATV systems for the extended frequency bandwidth and effectively larger signal variation, a weak signal level requires amplification with low noise to achieve satisfactory signal-to-noise ratio and an extended frequency bandwidth along with a high signal level requiring larger signal handling capability. Low noise amplifier transistors which are gain reducible with biasing changes are more subject to modulation distortion than bipolar transistors with high current operation and high f.sub.T with practically no changes in noise figure and gain over a large range of collector currents. To further improve the dynamic range without significantly degrading the signal-to-noise ratio, variable attenuation is required to avoid modulation distortion in the converter.
In the past, television receiver circuits have been proposed in which a controlled PIN diode network is coupled to the input between the antenna and the preamplifier (RF amplifier) transistor in order to improve the dynamic range. These PIN diodes have an intrinsic layer between the p-region and the n-region. For RF signal currents, the PIN-diode primarily behaves as a resistor whose value is dependent on the DC biasing of the diode. Examples of such circuits can be found in U.S. Pat. Nos. 3,800,229 to Backwinkel et al, 3,813,602 to Van Dijum et al, 4,019,160 to Kam and 3,577,103 to Sparks. Prior art RF receivers incorporating a PIN diode attenuator feature have been directed toward use between the receiver's antenna input and the tuner in combination with an external driver for the PIN attenuator and have not relied upon preamplifier bias changes to improve the dynamic range of the transistor itself. Also, the PIN diode attenuator has not been used in a CATV signal environment in the past where signal level variation is substantially less than that encountered when signals are transmitted "on air" and also for narrower bandwidth operation
U.S. Pat. No. 4,019,160 to Kam describes a signal attenuator circuit for a TV tuner where the PIN attenuator is used between the RF input and the RF amplifier, and also the AGC input voltage increases the input impedance of the transistor by reducing transistor current which reduces the noise slightly from the amplifier due to improved noise matching. However, the noise figure degradation is essentially in direct proportion to the attenuation achieved by the PIN diode attenuator and also, due to the AGC'able transistor amplifier, increased modulation distortion is produced compared with non-AGC'able high current transistors.
U.S. Pat. No. 3,800,229 to Backwinkel et al describes a gain controlled high-frequency input stage having a PIN diode network where a PIN-diode attenuator is used in front of the RF amplifier with the emitter voltage of the high current transistor serving as the reference voltage for the PIN diode network. U.S. Pat. No. 3,813,602 to Van Dijum et al discloses a PIN diode attenuator at the input of the RF amplifier. U.S. Pat. No. 3,577,103 to Sparks describes a variable attenuator for a wave signal receiver where a separate transistor is used to drive a PIN diode attenuator which is used at the RF input circuit between the receiver's antenna and the tuner's input.
Most CATV converters are those of the up/down conversion type where neither RF amplifier or variable attenuator circuits are utilized. This has been due to the fact that the frequency bandwidth has been only up to 300 MHz (35 channels) and the signal level over the frequency band over which these converters are utilized is more easily and accurately controlled.
One example of a CATV frequency converter for selectively providing access to only predetermined channels is disclosed in U.S. Pat. No. 4,079,415 to Will. This converter up-converts 50 to 300 MHz to 500 to 750 MHz and involves a single conversion converter. Due to insufficient selectivity at the second conversion which takes place in the UHF TV tuner, adjacent channel cross-modulation and intermodulation is excessive and unsatisfactory.
Another example of a frequency converter is set forth in U.S. Pat. No. 3,939,429 to Lohn et al which discloses double conversion in a television receiver tuner. The RF amplifier is external and there is no PIN diode attenuator between the RF amplifier and the first mixer.
Yet another example of a frequency converter is set forth in U.S. Pat. No. 3,801,915 to Ostuni et al which discloses double conversion in the CATV converter without either an RF amplifier or PIN-diode attenuator and the use of an active mixer. This system suffers from a high noise figure and is more susceptible to modulation distortion. U.S. Pat. No. 4,270,212 to Furukawa is directed to bypassing a CATV converter when the input channel is the same as the converter output frequency. The AGC circuit and attenuator are placed at the output of the CATV converter and do not improve either the noise figure or dynamic range of the converter. U.S. Pat. No. 4,352,209 to Ma also provides up and down frequency conversion in a CATV converter.
Therefore, in view of the above, the present invention provides a low noise CATV converter involving the upconversion and downconversion of a received CATV signal which is amplified at the input stage of the converter. Modulation distortion due to increased signal level at the mixer stage due to a low noise RF amplifier is compensated for by the unique application of a PIN diode attenuator and is further reduced by a unique biasing network for the RF amplifier transistor and PIN diode attenuator.